The present disclosure relates to multiphase generators and more specifically to multiphase generator-conversion systems and methods of mitigating harmonics in such systems.
Wind turbines have to comply with certain standards regarding the quality of the power they generate. These standards are typically adopted by the electrical network operators in their respective grid codes, in which they determine the technical requirements for wind facilities for their grid integration. According to some grid codes, the power quality evaluation includes, among other aspects, the evaluation of the harmonic components in current and voltage waveforms at the connection point of the wind facility with the electrical network. Harmonic components may be provoked by generator imperfections or may be generated by the converters at the conversion lines. The study of harmonics on the generator side is relevant for wind turbine facilities because, among other aspects, harmonic currents flowing through the windings of the generators may provoke mechanical vibrations, which can diminish the life of their mechanical parts. Furthermore, they can increase the electrical power losses affecting the efficiency of the machine. Moreover, from the point of view of the management and control of the wind turbine, they can provoke measurement errors and interferences in communication systems.
The harmonic emission in wind turbines can be affected, among others, by the following aspects: i) the use of electronic power devices; and ii) the electromechanical design of the generators. Typically, the desired voltage waveforms at the stator terminals of the generators, and also at the grid connection point of variable speed wind turbines, are obtained by the switching of electronic power converters. However, the obtained waveforms are not purely sinusoidal but contain harmonic components. Regarding the electromechanical design of the generator, it is worth highlighting that the design of the windings, the stator and the rotor of the generator may affect the magnetic flux within the generator and thus the circulation of harmonic currents through it, and from it to the power converters which connect the system with the external grid.
The problem of the harmonic emission is more pronounced in DC wind turbine power plants. DC wind turbine power plants are wind turbine power plants with a DC collection grid, instead of a conventional AC collection grid. In order to implement DC wind power plants using conventional multiphase AC wind turbine generators, the output of the wind turbines power conversion systems must be in DC. To do that, the extensive use of power conversion systems based on electronic power converters is required. Power converters typically affect both the harmonic emission from the wind turbine to the DC collection grid of the DC wind power plant, as well as the harmonic currents flowing through the electrical generator of the wind turbines.
The present disclosure relates to various methods and devices for avoiding or at least partly reducing this problem.